Method and system for controlling a separator unit for multiphase separation of fluids

ABSTRACT

A method and a system for controlling a separator unit for multiphase separation of fluids of different densities, wherein either a pressure in the separator unit or a level of one or more of the liquids in the separator unit is adjusted in relation to a reference value. The reference value and the relevant pressure in the separator unit ( 1 ), or the level of the relevant liquid converted to a pressure, are supplied to either side of a pressure sensitive mechanical device ( 23; 28 ) which moves with deviations of the pressure from the reference value, and the movement is transferred directly to a mechanical control unit ( 22; 33 ) which is coupled to a control device ( 20; 38 ) on a fluid outlet ( 25; 37 ) from the separator unit ( 1 ), and which utilizes the difference between the pressure in the separator unit and the pressure downstream of the control device for moving this in the desired direction for correcting for the deviation.

This application is a 371 of International Application No.PCT/NO00/00429, filed on Dec. 14, 2000, which designated the UnitedStates of America.

FIELD OF THE INVENTION

The invention relates to a method and a system for controlling aseparator unit for multiphase separation of fluids of differentdensities, wherein either a pressure in the separator unit or a level ofone or more of the liquids in the separator unit is adjusted in relationto a reference value.

BACKGROUND OF THE INVENTION

Today, separators in connection with offshore oil and gas extractiontraditionally are placed above water. However, for economical andextractional reasons, the development goes in the direction of aplacement on or below the sea bed. The advantages herewith in principlewill increase with an increasing ocean depth and an increasing distancefrom receiving plants for fluids from the separator, but simultaneouslythe complexity and costs associated with control and energy supply tothe individual control devices will increase.

The control of the separator units for multiphase separation of fluidsof different densities traditionally is carried out via complicated andresource-demanding, instrumented control loops. The conventional controlloops normally will comprise measuring of a parameter value, and signaltransmission to a regulator which compares the measured value with thereference value and decides the action which, in the form of a controlsignal, is sent back to the control device seeking to correct a possibledeviation. The control device requires energy, something which for thetraditional solutions involves an external supply via hydraulics,electric power or pneumatics. The most important regulating functions inconnection with separation plants will be control of gas pressure andlevel control of liquids. Relevant control devices for example will bemains frequency controlled pumps and process control valves.

The level control systems which today are qualified for installation inan under-water separation plant, are relatively large and bulky, and inaddition have a long response time. With increasing distances one willget an increased time delay. For relatively large separators one may beable to obtain a satisfactory stability of the respective parameters,even if the control loop is slow. For more compact separators theresponse time often will be significantly shorter, something which mayturn out to be difficult to obtain with the qualified level measuringprinciples of today.

The main object of the invention is to provide a method and a system forachieving a simple, precise, robust and energy-economical mechanicalcontrol loop for pressure and level control of fluids in connection withdifferent types of separators and tanks placed above or below sea level.

A more particular object of the invention is to provide a method and asystem wherein the control loop for the process is substantially simplerand quicker than the control loops in separator plants according to theprior art, at the same time as external energy supply can be limited topossible signal lines to electronic pressure sensors and solenoids.

SUMMARY OF THE INVENTION

The above-mentioned objects are achieved with a method of theintroductorily stated type which, according to the invention, ischaracterised in that the reference value and the relevant pressure inthe separator unit, or the level of the relevant liquid converted to apressure, are supplied to either side of a pressure sensitive mechanicalmeans which moves with deviations of said pressure from the referencevalue, and that the movement is transferred directly to a mechanicalcontrol unit connected to a control device on a fluid outlet from theseparator unit, and utilising the difference between said pressure inthe separator unit and the pressure downstream of the control device formoving the device in the desired direction for correcting for thedeviation.

According to the invention there is also provided a system of theintroductorily stated type, which system is characterised in that itcomprises a pressure sensitive mechanical means arranged to be suppliedwith the referent value and the relevant pressure in the separator unit,or the level of the relevant liquid converted to a pressure, to oppositesides of said means, and to be moved with deviations of said pressurefrom the reference value, and a mechanical control unit connected to themechanical means and to a control device on a fluid outlet from theseparator unit, and arranged to utilize the difference between saidpressure in the separator unit and the pressure downstream of thecontrol device for moving the device in the desired direction forcorrecting for the deviation.

In the method according to the invention there is established areference value in the form of a pressure, and this reference value iscompared with the actual pressure in the separator unit, or with theactual liquid level converted to a pressure. A possible deviation isconverted directly to a mechanical movement which in turn is coupled toa mechanical control unit arranged to utilize the difference between thepressure of the separator chamber and the pressure downstream of therespective control device, to establish the force required by thecontrol device in order to correct the deviation.

If the relevant well delivers gas, one will want to utilise the gas fromthe upper part of the separator tank as a driving medium for all thecontrol devices. This is due to the circumstance that the gas willcontain little contamination that may damage valve seals etc. It shouldotherwise be mentioned that the control devices will be able to beconstructed in such a manner that they require a very small supply ofdriving medium as compared to the fluid flow controlled by the finalcontrolling element. The driving medium therefore may be dumped into theprocess line downstream of the respective control devices without thishaving any practical influence on the separator functions.

It is necessary to construct the control devices such that a sufficientmanipulated variable force is obtained also when the difference betweenthe separator pressure and the pressure downstream of the relevantcontrol device is at the lowest level, for example when a finalcontrolling element in the form of a throttle valve is maximally open.This implies that one bases oneself upon pressure-balanced or littleforce-demanding valve devices, and that for possible other types ofdevices one provides for a sufficient manipulated variable force byincreasing the surface influenced by the driving medium.

If special circumstances should dictate that it is not appropriate touse a fluid from the separator tank as a driving medium, one couldalternatively utilise the pressure difference between a driving medium(e.g. hydraulic oil) supplied from the surface and the pressuredownstream of the relevant control devices. This will enhance the priceand complicate the operation of the separator, but one achieves theadvantages of the invention in the sense that a deviation gives anapproximately instantaneously correcting effect on the final controllingelement because the control loop is based on a direct-acting mechanicalconnection.

Mechanised regulating processes according to the invention are basedupon the actual value being compared with a reference value which is acombination of a spring tension and a pressure, the reference valuebeing changed in that one or both of these parameters are changed. Therewill relatively seldom be a requirement to change the reference valuesfor the liquid levels in the separator tank. It is more of interest tobe able to change the reference value for the pressure in the separatortank in a simple manner. For practical reasons one will preferablychoose to control this by means of electronic sensors. This is done byusing a valve device which is arranged to change the pressure componentof the reference value by means of a solenoid which alternativelyprovides for supply or discharge of fluid from a reference chamber inthe device. The advantages of this method are that it is simple tochange the pressure in the separator, at the same time as the externalenergy consumption for the regulation mainly is limited to a moderatecurrent consumption for the operation of pressure sensors and solenoids.The technology for establishing such a reference pressure will be knownto a person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below in connection withexemplary embodiments with reference to the drawings, wherein

FIG. 1 shows a schematic view of a conventional separator unit forseparation of the fluid from an oil well, whereby oil, gas and producedwater are carried in separate lines;

FIG. 2 shows a schematic view of an embodiment of a separator unithaving a control system according to the invention, wherein one haschosen to use natural gas from the separator unit as a driving mediumfor the final controlling elements;

FIG. 3 shows a schematic view of a preferred embodiment of a mechanicalcontrol unit for a control device in the control system according to theinvention; and

FIGS. 4 and 5 show an embodiment of a control device for use in thesystem according to the invention, this being specially constructed forcooperation with the mechanical control unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the main elements of a conventional separation plant forthrees phase separation of a fluid from an oil well. The plant comprisesa separator unit which is situated on or at the sea bed, and whichcomprises a tank 1 having an inlet 2 for the fluid from the oil well.The tank defines a separator chamber 3 comprising an upper or firstchamber part 4 and lower, second and third chamber parts 5 and 6 whichare separated from each other by a barrier or partition 7 projectingfrom the bottom of the separator chamber.

Gas will fill up the upper chamber part 4, whereas oil and water runinto the second chamber part 5 where the difference in specific weightcauses the water to sink to the bottom. Oil from the chamber part 5 willrun over the barrier 7 and into the third chamber part 6. In a normaloperating situation one wants to maintain as stabile operatingconditions as possible. This implies that one seeks to control thedischarge of the different fluids such that the pressure in theseparator tank and the level of oil and water, respectively, in therespective chamber parts is as stabile as possible. In the illustratedexample this is done by means of a pair of valve devices (controldevices) 8, 9 and 10, 11 and a pump 12 which are controlled by theirrespective control units.

The valve device 8, 9 comprises an actuator 8 and a throttle valve(final controlling element) 9 which, controlled by its control unit (notshown) in cooperation with an electronic pressure sensor 12, regulatesthe pressure in the chamber part 4 and thereby the gas delivery from theseparator.

Correspondingly, the valve device 10, 11 comprises an actuator 10 and athrottle valve 11 which, controlled by its control unit (not shown) incooperation with a level gauge 13, regulates the oil level in thechamber part 6 and thereby the delivery of oil from the separator.

The pump 12 shall, controlled by its control unit (not shown) incooperation with a level gauge 14, see to it that produced water isconducted away from the separator unit. Starting from the levelmeasurement, the control unit of the pump will control the admission ormanipulated variable of the pump so that the flow from the water outlet15 of the separator tank is changed and the water level in the chamberpart 5 is maintained at the desired level. The level control may bebased on the admission of the pump being varied by the use of a mainsfrequency transformer. An alternative may be based on a mechanicaltransfer for controlling a viscose coupling between the pump and itsmotor. One may also manipulate the flow rate through the pump by meansof a variable throttling upstream and/or downstream of the pump.Further, one may base oneself upon the pump having a fixed rotationalspeed and that the admission or manipulated variable is changed by meansof a controlled leakage, for instance between the downstream and theupstream side of the pump. The leakage may also be carried to otherparts of the process plant, or a fixed quantity may be recirculated anda variably quantity vented out of the system.

The above-mentioned control units for the valve devices and the pumpform part of the control system of the separator unit situated at thesea level. The interface X in FIG. 1 illustrates how the respectivemeasuring signals are transferred to the control system at the surface,and how an externally supplied driving medium is supplied to the controldevices, whereafter in given cases it is returned to the energy source.This requires a complicated control system having a significant numberof signal lines and pneumatic/electronic/hydraulic lines to therespective control devices.

A separator unit which is provided with a control system according tothe invention is shown in FIG. 2.

The separator tank proper corresponds to FIG. 1, and corresponding partsin the two figures consequently are designated by the same referencenumerals.

In a manner corresponding to that of FIG. 1, the regulation of thepressure in the chamber part 4 is based on a valve device comprising anactuator 8 and a final controlling element in the form of a throttlevalve 9 which, controlled by a control unit 16, regulates the pressurein the chamber part 4 by varying the gas delivery from the separatorunit. The control unit 16 compares the actual gas pressure in theseparator tank with a reference pressure, and sees to it that thethrottle valve is set in a position wherein the pressure is maintainedstable at the desired level.

The control unit 16 comprises a reference chamber (not shown) which, bymeans of a solenoid 17, may be connected to a gas inlet 18 at theupstream side of the throttle valve 9 or alternatively to a gas outlet19 at the downstream side of the valve, so that the reference pressuremay be raised or lowered dependent on the position of the solenoid. Thesolenoid is connected or disconnected by means of control signals fromthe surface, supplied via the interface Y. The control signals aredependent on the actual gas pressure which is sensed by means of theelectronic pressure sensor 12.

The oil level in the chamber part 6 of the separator tank 1 and thewater level in the chamber part 5 are controlled by means of respectivemechanical control means which are constructed in accordance with theinvention, and which will be further described below.

The oil level in the chamber part 6 is maintained stable by means of avalve device 20, 21 in cooperation with the mechanical control unit 22.As further described in connection with the embodiment shown in FIG. 3,the arrangement comprises a pressure sensitive mechanical means 23comprising a diaphragm arranged such that it registers the hydrostaticpressure difference between the gas in the separator tank 1 and the oilat a given level in the chamber part 6. This pressure difference isregistered via a line 24 from the upper chamber part 4 and an oil outletline 25 from the chamber part 6, and is a measure of the oil level inthe chamber part 6. The desired oil level is set by arranging a springwhich influences the pressure sensitive diaphragm in the same directionas the gas pressure. The desired oil level then is defined as the levelcorresponding to the fact that the hydrostatic pressure of the oil justmanages to balance the force from the spring and thereby places thesensor diaphragm in a neutral position, defined by the fact that thereis no fluid flow in the mechanical control unit 22.

As appears from FIG. 2, the gas outlet line 24 is also connected to themechanical control unit 22, so that the natural gas from the separatorunit is used as a driving medium for the control unit. The control unitfurther comprises a discharge line 26 for the driving medium, and thisline is connected as shown to an oil outlet 27 at the downstream side ofthe final controlling element 21 of the valve device 20, 21. Thissuitably consists of a throttle valve, as further described inconnection with FIGS. 3-5.

The control of the water level in the chamber part 5 is based on thesame principle, but requires a somewhat more complicated device as aresult of the fact that, in the chamber part 5, there are two liquidphases of which the level of one phase is to be regulated. This is donein that a pressure sensitive sensor diaphragm in a mechanical means 28which forms part of the arrangement, senses the hydrostatic pressuredifference between a first liquid column having a defined specificweight influencing the upper side of the sensor diaphragm, and a secondliquid column consisting of water and oil influencing the underside ofthe sensor diaphragm. The pressure from the first liquid column issupplied via a first outlet pipe 29 from a chosen level in the chamberpart 5 below the upper edge of the barrier 7, whereas the pressure fromthe second column is supplied via a second outlet pipe 30 from thechamber part 5 at the bottom of the separator tank 1. The pressure onthese to columns is identical from a common reference point 31 andfurther up to the gas-filled volume 4 in the tank. Since water has ahigher density than oil, the pressure influencing the underside of saiddiaphragm will increase with an increasing water level in the tank. Thepressure difference which is sensed, therefore is a measure of the waterlevel in the tank. In a manner corresponding to that of adjusting theoil level, the water level is adjusted by arranging a spring whichinfluences the pressure sensitive diaphragm in a direction causing it tobe in a neutral position at the desired water level.

For ensuring that the density of the water column between the upper sideof the sensor diaphragm and the reference point 31 is not changed, apreferred embodiment is based upon this liquid column being locked inbetween said sensor diaphragm and a compliant diaphragm (not shown)arranged in a flange 32 inserted into the outlet pipe 29 between thesensor diaphragm and the separator tank 1.

The pressure sensitive means 28 cooperates with a mechanical controlunit 33 which is of a similar construction as the control unit 22, andwhich is driven by the same driving medium as this unit, i.e. the gasfrom the separator tank 1. Thus, the supply line 24 for driving mediumto the control unit 22 is coupled to a supply line 34 for driving mediumto the control unit 33, whereas a discharge line 35 from the controlunit 33 is coupled to the discharge line 26 coming from the control unit22 and being connected to the oil outlet 27 at the downstream side ofthe final controlling element 21.

In the embodiment according to FIG. 2, the water delivery from theseparator tank 1 is controlled by means of a pump 36 which is connectedto the chamber part 5 via an outlet pipe 37. The outlet of the pump iscontrolled by the control unit 33 via a control device in the form of avalve device 38, 39 comprising an actuating drive in the form of anactuator 38 affecting a final controlling element in the form of athrottle valve 39. As further described in connection with FIG. 3, thecontrol unit 33 includes two outputs which are connected to the actuator38 via respective lines 40 and 41.

It is to be emphasised that FIG. 2 is a pure principle drawing. Forexample, the above-mentioned level control for the water will be basedon measuring points situated quite close to the barrier 7, and not justbelow the inlet 2 for the fluid in the oil well. In this region therewill be strong whirls preventing oil and water from being separated, andin addition resulting in unstable pressure conditions. Differentseparator tanks may have different internal divisioning and a differentnumber of inlet and outlet pipes. It is also to be empasised that themechanical control means according to the invention may be adapted toother types of final controlling elements than for example throttlevalves.

The following calculation example shows the forces which may begenerated via the sensor diaphragm in order to control the regulation:

It is supposed that the diaphragm has an effective pressure surfacecorresponding to D=150 mm, which gives an area equal to 177 cm². It isfurther supposed that the water level departs with 5 cm from the desiredvalue. The density of the oil typically will be about 0.7 g/cm³, whereasthe density of water is 1 g/cm³. If 5 cm of the water column is replacedby oil, the pressure change will correspond to (1-0.7)×5 ponds/cm²=1.5ponds/cm². This implies that the diaphragm is subjected to a correctingforce of K=177×1.5 ponds=265 ponds.

A mechanical control unit according to the invention needs asubstantially smaller force than this in order to be able to establish amaximum manipulated variable or admission which, in principle, isdetermined by the difference between the pressure in the upper part ofthe separator tank and the pressure at the point where the drivingmedium is dumped. If, for example, one presupposes that this pressuredifference is 2 bar, one will obtain, against a pressure surfacecorresponding to 100 cm², a maximum manipulated variable forcecorresponding to 200 kponds. Relevant control devices may be constructedso that they will be able to operate with a substantially smaller forcethan this.

In the separator unit according to FIG. 2 one has chosen to control thewater discharge by means of an electric pump 36 carrying the water to adeposit at the surface. Further, one has chosen to stabilise the waterlevel by coupling the mechanical control unit 33 and the valve device38, 39 in a manner causing an increasing throttling of the outlet of thepump with a decreasing water level, and vice versa. In this situationthe downstream pressure of the pump is higher than the pressure in theseparator tank 1. In order to have a sufficient force to operate thecontrol device 38, 39, it is therefore appropriate to dump the drivingmedium downstream of the valve device 20, 21, as shown in FIG. 2, orpossibly downstream of the valve device 8, 9.

FIG. 3 shows a preferred embodiment of the mechanical control unitforming part of the system according to the invention. The arrangementis here adapted to an embodiment for stabilising the water level in thechamber part 5 of FIG. 2.

As shown, the arrangement comprises a pressure sensitive mechanicalmeans 50 comprising a housing 51 in which there is arranged a diaphragm52 which is preloaded by means of a spring 53. The housing 51 has alower inlet 54 communicating with the underside of the diaphragm 52, andan upper inlet 55 communicating with the upper side of the diaphragm.The lower inlet 54 here is intended for connection to the outlet pipe 30of the separator tank 1, whereas the upper inlet 55 is intended forconnection to the outlet pipe 29 of the tank. During operation theunderside of the diaphragm 52 then will sense the liquid pressure at thebottom of the chamber part 5, whereas its upper side will sense thepressure from the liquid-filled outlet pipe 29. Deviations in the waterlevel in the chamber part 5 in relation to the desired level then isregistered as a displacement of the sensor diaphragm 52.

In accordance with the invention, and as stated in the introduction, themovement of the pressures sensitive means, i.e. the sensor diaphragm 52,is transferred directly to a mechanical control unit which is connectedto a control device on a fluid outlet from the separator unit, and whichutilises the difference between the pressure in the separator unit andthe pressure downstream of the control device for moving this in thedesired direction in order to correct for the sensed deviation. For thispurpose the sensor diaphragm 52 in the illustrated embodiment is coupledto one end of a transfer arm 56 of which the other end as shown isformed with a centrally supported lever 57 which is arranged tocooperate with a valve device forming part of the mechanical controlunit. This unit in its entirety is designated by the reference numeral60 and comprises a housing 61 in which said valve device and theremaining elements of the unit are arranged.

The valve device comprises first and second valve bodies 62 and 63,respectively, which are arranged to be influenced by respective ends ofthe lever 57, the first valve body 62 being coupled to the lever via acylinder-shaped piston 64 which is slidably arranged in the housing 61as shown in FIG. 3. The valve bodies 62, 63 control inflow and outflowof driving medium respectively to and from a valve chamber 65, asfurther described below.

As mentioned above, the unit 60 is arranged to influence a controldevice arranged on a fluid outlet from the separator tank 1. Thepressure forces used for establishing the necessary energy for themanipulated variable, are provided in that the upper chamber part 4 ofthe separator tank and the downstream side of the control device(throttle valve) in question are connected to a respective one of twochannels or ducts 66 and 67 arranged in the housing 61. Thus, in theillustrated case, the duct 66 is presupposed to be connected to the line24 (FIG. 2) via a pipe connection 68, whereas the duct 67 is connectedto the line 26 via a pipe connection 69. The ducts 66 and 67 may becommunicated with respective ones of a pair of outputs or exits 70 and71 from the unit 60, as further described below.

The driving force for the relevant control device is taken from thepressure difference established between the exits 70 and 71. Untilpossibly having a maximum manipulated variable, this pressure differencewill increase or decrease proportionally to a change in the net forcetransferred from the sensor diaphragm 52 to the transfer arm 56. In theillustrated embodiment the control unit is constructed such that evenmaximum variations in the manipulated variable requires a smalldiaphragm movement. Further, the is control unit is provided with avalve system 72-74, 75-77 causing the control unit to have the capacityto provide for rapid correcting movements of the final controllingelement, even if it should be necessary to use a liquid instead of gasas the driving medium.

It is desirable that the mechanical control unit 60 has stable workingconditions, and it is therefore not very favourable that the pressuredifference between the ducts 66, 67 may vary strongly. In order to avoidthat the control unit becomes less precise because of these variations,the unit is provided with a pressure-stabilising valve means 77-81seeing that the pressure difference between the inlet duct 66 and thechamber 77, which has an open connection to the exit 71, is kept at astable level. This pressure difference in the reality is the workingpressure for the mechanical control unit, and corresponds to the maximumpressure drop which may be established between the exits 70 and 71. Thepressure difference which is maintained by means of thepressure-stabilising valve means, consequently must be chosensufficiently high that the manipulated variable that can be established,at any time is sufficient in relation to the process which is to becontrolled. Said pressure difference may not be larger than the pressuredifference applying at any time between the separator chamber and thedownstream side of the throttle valve in question. It is thereforeimportant to secure that the pressure drop across the throttle valvenever becomes to low. With a correct dimensioning of the throttle valvesin relation to the piping connecting the valve in question with thereceiving plant for the relevant fluid, one will be able to secure thatthese always have a minimum pressure drop which can be used foroperation of control devices. This will be further elucidated below.

The pressure stabilising valve 77-81 comprises a sensor diaphragm (or apiston) 80 cooperating with a valve body 78. The sensor diaphragm isinfluenced by a spring 79 having a given preload actuating the diaphragmin the downwards direction. This implies that the valve body 78 will bepressed away from its seat if there is not a sufficiently large pressuredifference between the chambers 81 and 77 below and above the sensordiaphragm. In that case this will open for a certain gas flow from thechamber 77, via the duct 67 to the relevant downstream course where thegas is dumped. The gas flow ceases only when the pressure in the chamber77 again becomes so low relative to the pressure in the chamber 81 thatsaid spring preload does not manage to keep the valve body 78 away fromits seat. The pressure difference between the chambers 81 and 77therefore will be stabilised at a level which will be determined by thepreload of the spring 79, and this corresponds, as previously mentioned,to the maximum pressure difference which can be transferred to the finalcontrolling element to be operated by this control unit.

In FIG. 3 the sensor diaphragm 52 is shown in a neutral position. Thisimplies that the valve bodies 62, 63 rest against their respectiveseats. When the sensor diaphragm 52 is pressed upwards as a result of anincreased water level in the chamber part 5, the lever 57 will press thepiston 64, and therewith the valve body 62, to the right, so that thereis opened for a gas supply from the inlet duct 66 to the chamber 65.This implies a pressure increase in this chamber which will influencethe end surface of the piston 64 with a force which is directedoppositely to the force from the lever 57. At the same time the pressureincrease will press the valve pistons 72 and 75 to the right. The valvebody 73 thereby is pressed with an increased force against its seat,whereas the valve body 76 is pressed away from its seat. This opens forgas flow from the inlet duct 66, via the chambers 77 and 74, to the exit70. The valve body 76 has a large cross-section, and the pressure at theexit 70 will rise rapidly to approximately the same level as in thechamber 65, even if the final controlling element in question shouldrequire a relatively large fluid supply for its function. The pressurein the relatively small chamber 65 will rise rapidly to a level at whichthe force which influences the right end surface of the cylinder piston64, balances the oppositely directed force from the sensor diaphragm 52via the transfer arm 56. This implies that the sensor diaphragm ispressed back to the neutral position, so that the valve body 62 comes torest against its seat and blocks for a further pressure buildup in thechamber 65, and thereby also for a further pressure buildup on the exit70. The total pressure increase on the exit 70 in relation to the exit71 by this will be approximately proportional to the increase of theforce generated by the deviation against the sensor diaphragm 52.

When the deviation force against the sensor diaphragm 52 decreases, thediaphragm will be pressed downwards in relation to the neutral position.This is a consequence of the fact that the upwards directed deviationforce becomes smaller, whereas the oppositely directed pressure forcesfrom the chamber 65 against the right end surface of the cylinder piston64 are still equally large, since the cylinder 64 is not connected tothe valve body 62. This entails that the lever 57 now will press thevalve body 63 away from its seat, so that there is opened for a quickventing from the chamber 65 to the exit 71 until the pressure forcesagainst the right end surface of the cylinder 64 are reduced so that thevalve body 63 again closes against its seat.

With a falling pressure in chamber 65, the pressure at the exit 70 isimmediately correspondingly reduced, since the pressure differencebetween the chambers 74 and 65 will push the piston 72 to the left, andthereby vent excessive pressure in that the valve body 73 for a shortmoment opens a duct having an open connection to the exit 71.

In a corresponding manner as with increasing deviation forces, alsodecreasing deviation forces will entail that the pressure differencebetween the exits 70 and 71 decreases proportionally to the reduction inthe force generated by the deviation against the sensor diaphragm 52. Inthe illustrated embodiment of the control unit according to theinvention one will therefore be able to maintain an approximately linearconnection between deviation and manipulating variable force (up to amaximum manipulated variable).

As shown in FIG. 3, the control unit 60 is also provided with a valve inthe form of a spring-loaded valve body 82 arranged in a duct between theinlet duct 66 and the valve chamber 65. This duct communicates with thechamber 77 via a passage, as shown in the figure. It is here thequestion of a “short-circuit” valve which is arranged to cause areduction of the driving medium pressure if this should become too high.

FIG. 3 also shows how the control unit 60 may be connected to anactuating drive in the form of an actuator 90. This is here shown tocomprise a spring-loaded pilot valve 91 which is arranged to influence afollowing, non-illustrated final controlling element. To obtain optimumregulating conditions, it is preferable that the actuating drivecooperates with the final controlling element (throttle valve) inquestion, so that this adjusts itself to a position which isunambiguously determined from the manipulated variable. For example, ifone wants to control a viscose coupling between a motor and its pump,the manipulated variable of the actuator should meet a counter forcewhich increases uniformly with an increasing pump power. As an example,there is taken as a starting point that the relevant final controllingelements are dimensioned for a maximum manipulated variable pressureequal to 2 bar, and that one wants to utilise the difference between thepressure in the upper part of the separator chamber and the pressuredownstream of the respective valve devices.

The pressure drop across a throttle valve is always smallest when thevalve is in the quite open position, and in this position the pressuredrop across the valve will constitute a fraction of the total pressuredrop in the piping between the separator tank and the receiving plant.By choosing a correct dimensioning of the throttle valves in relation tothe piping, the pressure drop across the valves during normal operationwill not be able to be less than for example said 2 bar. In somesituations the final controlling element will be able to control valvedevices wherein the downstream pressure is not suitable to the purpose.Examples hereof is the control of the manipulated variable of the pumpas shown in FIG. 2. The downstream pressure of the pump 36 here ishigher than the pressure in the separator tank. It will here be ofcurrent interest to dump the driving medium for the final controllingelement into the downstream course for one of the remaining throttlevalves, for example downstream of the throttle valve 21.

FIGS. 4 and 5 show schematic sectional views of a throttle valve whichhas been developed especially for cooperating with a mechanical controlunit according to the invention. The valve comprises a valve housing 95having an inlet 96 and an outlet or downstream course 97. Between theinlet and the downstream course there is provided a piston 98 which isslidably arranged in an outer sleeve 99 and the end cover 100 thereof.The piston has an axially through-going opening 101 and is designed suchthat the pressure in the downstream course 97 of the throttle valve isbalanced against the end surfaces of the piston, so that there is notproduced any force influence worth mentioning on the piston in the axialdirection.

The piston 98 is formed with an outer peripheral flange 102, so that alower chamber 103 is formed between the flange and the bottom of thesleeve, and an upper chamber 104 is formed between the flange and alower end surface of the end cover 100 of the sleeve. These chamberscommunicate with respective channels 105 and 106 for the supply ofdriving medium from respective ones of the exits 70 and 71 of thecontrol unit 60, as suggested at the top of FIG. 4.

As shown, a relatively strong spring 107 is placed between an innerabutment flange in the piston 98 and an abutment surface on the endcover 100 of the sleeve. The spring 107 at any time will seek to pressthe piston 98 downwards towards its seat 108. In order to lift thepiston up from this seat, it is required that a pressure is establishedin the chamber 103 which is so much higher than the pressure in thechamber 104 that one gets a lifting force exceeding the spring tension.At a given minimum pressure difference between the two chambers, thepiston 98 will be lifted upwards to a position which is determined inthat the spring 107 has been compressed such that the spring tension isequally large and directed oppositely to the pressure forces. When thepiston is lifted up from the seat 108, there is opened for a downwardsdirected fluid flow. This will pass along the outside of the sleeve 99,through ports 109 arranged in the sleeve, past the annular openingformed between the piston 98 and the seat 108, to pass thereafterapproximately axially through the downstream course 97 of the throttlevalve.

The mechanical control unit 60 (FIG. 3) establishes the driving pressurebetween the exits 70 and 71. This is transferred via the channels 105and 106 to the chamber 103 and the chamber 104, respectively. The duct67 of the control unit is connected to the downstream course 97 of thethrottle valve via the shown channel 110. Provided that the gaskets 111,112 and 113 between the piston and the sleeve/end cover areleakage-free, all the consumed driving medium is dumped via the channel110. The system has no inherent leakage, which implies that drivingmedium is consumed only when the piston changes position. Provided thatthe piston 98 is placed vertically and higher than the downstream course97, the gaskets of the valve will be little subjected to problems as aconsequence of contaminations in the through-flowing fluid. Thisprobably functions optimally when using gas as a driving medium, since agas buffer then will be established within the piston 98. It shouldotherwise be mentioned at the valve housing 95 does not have to beshaped as a 90° bend as shown in FIGS. 4 and 5. The valve functionsequally well if the valve housing for example is shaped as a straightpipe.

In given cases it may be necessary to open up or throttle the fluidsupply to the separator tank. This especially applies if pressure orlevel parameters have come outside of acceptable values. For thispurpose one may advantageously apply a valve device which is based onthe same driving medium as the above-mentioned valve devices, thedriving medium consumed by this valve device being dumped in thedownstream course for one of the remaining valve devices.

What is claimed is:
 1. A method for controlling a separator unit formultiphase separation of fluids of different densities, wherein either apressure in the separator unit or a level of one or more of the liquidsin the separator unit is adjusted in relation to a reference value,characterised in that the reference value and the relevant pressure inthe separator unit, or the level of the relevant liquid converted to apressure, are supplied to either side of a pressure sensitive mechanicalmeans which moves with deviations of said pressure from the referencevalue, and that the movement is transferred directly to a mechanicalcontrol unit connected to a control device on a fluid outlet from theseparator unit, and utilizing the difference between said pressure inthe separator unit and the pressure downstream of the control device formoving the device in the desired direction for correcting the deviation.2. A system for controlling a separator unit (1) for multiphaseseparation of fluids of different densities, comprising a means foradjusting a pressure in the separator unit (1) or a level of one or moreof the liquids in the separator unit in relation to a reference value,characterised in that it comprises a pressure sensitive mechanical means(23; 28; 50) arranged to be supplied with the reference value and therelevant pressure in the separator unit (1), or the level of therelevant liquid converted to a pressure, to opposite sides of saidmeans, and to be moved with deviations of said pressure from thereference value, and a mechanical control unit (22; 33; 60) connected tothe mechanical means (23; 28; 50) and to a control device (20; 38; 90)on a fluid outlet (25; 37) from the separator unit (1), and arranged toutilize the difference between said pressure in the separator unit andthe pressure downstream of the control device for moving the device inthe desired direction for correcting the deviation.
 3. A systemaccording to claim 2, characterised in that the mechanical means (50)comprises a pressure sensitive element (52) which is coupled to atransfer element (56, 57) arranged to cooperate with a first valvedevice (62-64) in the mechanical control unit (60), the valve device(62-64) when influenced by the transfer element (56, 57) being arrangedto open for the supply of a driving medium to a valve chamber (65), forproducing a counter force on the transfer element (56, 57), so that thepressure sensitive element (52) is returned to a neutral position.
 4. Asystem according to claim 3, characterised in that the valve device(62-64) comprises a first and a second valve body (62 resp. 63) of whichthe first valve body is arranged to control inflow of driving mediumfrom an inlet duct (66) to said valve chamber (65), and the second valvebody (63) is arranged to control outflow of driving medium from thevalve chamber (65) to a first exit (71) from the control unit (60), thecontrol unit comprising an additional valve device (72-77) forconnection of the inlet duct (66) to a second exit (70) from the controlunit (60) under the control of the pressure in said valve chamber (65).5. A system according to claim 4, characterised in that the additionalvalve device comprises a first valve unit (75, 76) which is arranged tointerconnect the inlet duct (66) of the control unit (60) and its secondexit (70) at a given pressure in said valve chamber (65), and a secondvalve unit (72, 73) which is arranged to interconnect the two exits (70,71) of the control unit (60) at a pressure reduction in the valvechamber (65).
 6. A system according to claim 4, characterised in thatthe control unit (60) is provided with a pressure-stabilising valvemeans (77-81) seeing that the pressure difference between the inlet duct(66) of the control unit (60) and said first exit (71) is maintained ata stable level.
 7. A system according to claim 4, characterised in thatthe transfer element (56, 57) comprises a rotatably mounted lever (57)which, with its end portions, is arranged to influence respective onesof the valve bodies (62, 63) of the first valve device.
 8. A systemaccording to claim 5, characterised in that the control unit (60) isprovided with a pressure-stabilising valve means (77-81) seeing that thepressure difference between the inlet duct (66) of the control unit (60)and said first exit (71) is maintained at a stable level.
 9. A systemaccording to claim 5, characterised in that the transfer element (56,57) comprises a rotatably mounted lever (57) which, with its endportions, is arranged to influence respective ones of the valve bodies(62, 63) of the first valve device.
 10. A system according to claim 6,characterised in that the transfer element (56, 57) comprises arotatably mounted lever (57) which, with its end portions, is arrangedto influence respective ones of the valve bodies (62, 63) of the firstvalve device.
 11. A system according to claim 8, characterised in thatthe transfer element (56, 57) comprises a rotatably mounted lever (57)which, with its end portions, is arranged to influence respective onesof the valve bodies (62, 63) of the first valve device.